Molecular absorption by the dust lane and nuclear region of Centaurus A

VLA observations of the absorption spectra of OH, H2CO, and NH3 against the nuclear continuum radio source in Centaurus A are presented. Absorption has been detected near the systemic velocity by several molecules against the nucleus of Cen A and in H2CO at 6 cm against the inner jet. These results are consistent with absorption by one or more modest-sized molecular clouds in the dark lane. Absorption by OH is detected in the redshifted component believed to be associated with gas infalling into the nucleus. There is absorption in the red wing of the 2 cm transition of H2CO and possibly in the 6 cm transition against the nucleus and the jet. There are significant differences in velocity structure between OH and H2CO in the redshifted gas, and there appears to be no absorption at these velocities in some other molecules. The results for the infalling gas suggest that some molecules are dissociated by shocks and/or high-energy photons in the nucleus. 27 refs